A. Field of the Invention
The present invention relates to a new and improved refractory choke for a high intensity combustor; and more particularly, to a new and improved choke of refractory material including elements for improving the thermal conductivity of the refractory material and to a new and improved method for improving the combustion of a boiler.
B. Description of the Background Art
Boilers and steam generators typically include a combustor. A fire pot is included in the combustor with a throat at one end and a choke fabricated of refractory material at a second end. Combustion occurs in the fire pot and in high intensity combustors, temperatures in the fire pot reach 2500.degree. F. During combustion, recirculation of gases occurs in the fire pot and hot gases pass through the choke to a part of the boiler including a coil bank or heat exchanger. One side of the choke is exposed to high temperature combustion and the other side, outside the fire pot, experiences lesser temperatures of 200.degree. F. The massive refractory choke has a very low thermal conductivity of about 1.8 BTU/hr. ft..sup.2 .degree.F./ft. This combination of high temperatures and low thermal conductivity causes very high thermal gradients which in turn cause cyclic thermal strain resulting in destructive structural failure or cracks throughout the refractory choke.
In addition to cracks, surface spalling and disintegration of the face of the choke facing the interior of the fire pot occurs. This destruction of the face of the choke weakens and sometimes destroys recirculation in the fire pot. Since reduced recirculation results in a greater proportion of unburned fuel particles, the weakened or destroyed recirculation substantially decreases the efficiency of the boiler.
Present practice has been to reduce the flame temperature gradients to avoid damage to the choke. Reduction of flame temperature is typically accomplished by using larger amounts of combustion air. This procedure, however, reduces combustion efficiency and increases the formation of pollutants that are discharged into the atmosphere.
In the area of reinforced materials, there have been efforts to mix steel pins into concrete for the purpose of strengthening the structure formed of the concrete. Examples of reinforced concrete to prevent cracking due to the application of tensile or thermal stress are disclosed in U.S. Pat. Nos. 3,429,094 and 3,500,728. These patents are directed to strength enhancement and do not improve performance of a combustion system or improve thermal conductivity.